Mixer cooler

ABSTRACT

A mixer cooler having a mixing tool operating in a double walled mixing vessel, the cavity between the walls of the mixing vessel being subdivided by guide elements into a coolant duct which extends in labyrinthine fashion between a coolant inlet and a coolant discharge.

llnited States Patent [191 Langenberg 1 Mar. 18, 1975 1 MIXER COOLER[75] Inventor: HartmutLangenberg,Buer,

Germany [73] Assignee: Dierks & Sohne, Osnabruck,

Germany [22] Filed: Sept. 7, 1972 [21] Appl. No.: 286,949

[30] Foreign Application Priority Data Sept. 17, 1971 Germany 2146611[52] US. Cl 165/109, 165/169, 259/DIG. 18 [51] Int. Cl F28f 13/12 [58]Field of Search 165/109, 169; 259/D1G. 18

[56] References Cited UNITED STATES PATENTS 1,693,249 11/1928 Pauly165/169 1,922,534 8/1933 Ellsworth et a1 165/169 1,992,988 3/1935Blahnik 165/109 X 2,214,344 9/1940 Paul 165/169 X 10/1943 Higley 165/109X 2,545,371 3/1951 Mojonnier et a1. 165/169 X 2,557,622 6/1951 2,602,6487/1952 3,099,315 7/1963 3,554,274 l/1971 3,565,168 2/1971 Powell et a1.165/169 X FOREIGN PATENTS OR APPLICATIONS 635,970 9/1936 Germany 165/169212,607 8/1906 Germany 165/169 703,374 2/1954 United Kingdom 219/535Primary Examiner-Albert W. Davis, Jr. Assistant Examiner-S. J. Richter[57] ABSTRACT A mixer cooler having a mixing tool operating in a doublewalled mixing vessel, the cavity between the walls of the mixing vesselbeing subdivided by guide elements into a coolant duct which extends inlabyrinthine fashion between a coolant inlet and a coolant discharge.

10 Claims, 11 Drawing Figures [I 257 if a-mmin 1 8 s SHEET 2 UP 5 fig.)

MIXER COOLER The present invention relates to a mixer-cooler of the kindhaving a mixing vessel and a mixing tool arranged therein and designedto rotate about a vertical axis coinciding with the vessel axis, themixing vessel being a double-walled vessel whose internal and externalwalls define between them a space through which a coolant flows", insaid space guide elements being provided to guide the coolant on itspassage between a coolant inlet in the base and a coolant dischargemeans in the neighbourhood of the vessel rim.

In mixer-coolers of this kind that have been proposed heretofore, theguide elements arranged in the cavity of the double wall were employedto form a by-pass flow between the coolant inlet and the coolantdischarge and thus cut off parts of the double wall from the coolantflow. The normally solid, e.g., forged and machined, mixer tools extendradially from a hub mounted on a drive shaft, and generally take theform of a multiple beater tool set. As they rotate, they cause thematerial being mixed to adopt a funnel-shaped flow pattern in the courseof which the material, which at the same time is driven with a giratorymotion about the vessel axis, rises up the internal wall of the mixingvessel, out of the plane of operation of the mixing tools and, reversingits direction of motion downwards and inwards, returns to the zone ofoperation of the mixing tools again. Because it moves up overthe coolinternal wall of the mixing vessel, the material being mixed graduallydissipates its heat.

The cooling performance of mixer-coolers of this kind with heat transferthrough the vessel wall, is unsatisfactory. To improve the coolingperformance, therefore, in accordance with an older proposal, a coolinggas is blown into the mixing space to provide direct heat exchange inaddition to the existing indirect heat exchange.

The object of the present invention is to improve the coolingperformance of mixer-coolers of the kind above described, in terms oftheir indirect heat exchange, without incurring anyappreciable extracost of construction.

The present invention is a mixer-cooler, consisting of a mixing vesseland a mixing tool arranged therein and designed to rotate about avertical axis coinciding with the axis of the vessel, the mixing vesselbeing a doublewalled vessel whose internal and external walls definebetween them a jacket or space through which a coolant flows, in saidspace guide elements being provided to guide the coolant on its passagebetween a coolant inlet in the base and a coolant discharge means in theneighbourhood of the vessel rim. The guide elements subdivide the jacketinto a coolant duct extending in labyrinthine fashion from the coolantinlet to the cool ant discharge. In this context, at least in theradially outermost part of the base cavity of the double wall, the flowduct may consist of concentric duct sections constituted by guidebaffles extending substantially concentrically to the vessel axis; eachof which duct sections extends over an angle subtended at the centre ofaround 360 and is connected through a radial transfer orifice with theparticular adjacent, radially outer duct section, in a seriesarrangement. Furthermore, the flow duct may be formed in the side-wallzone of the double wall, by duct sections formed by guide bafflesdisposed coaxially and at intervals, parallel one above the other in thecavity, each of which duct sections extends through an angle subtendedat the centre, of 360, and

communicates through an axial transfer orifice with the particularadjacent axially upper duct section in a series arrangement. With thiskind of design, the coolant is made to follow a precisely defined flowpath, uniformly involving the entire vessel, as a consequence of whichthe formation of unwanted temperature differences in certain zones ofthe internal wall, is effectively avoided. The temperature of theinternal wall can be readily and accurately controlled with this kind offlow guidance, and a high degree of uniformity can be attained. Even atrelatively low rates of coolant flow it is possible, due

to the effectiveness of the heat transfer which can be achieved betweenthe internal wall of the double wall structure and the coolant, toattain at the internal wall of the vessel comparatively lowtemperatures, very little in excess of that of the coolant itself.

A further substantial increase in cooling performance on the part ofmixer-coolers of this kind can be achieved by designing the mixer toolsso that the lead ing face, considered in the direction of rotation ofeach tool, has an alignment which is tangential to the peripheralsurface of the hub. Preferably, in this embodiment, the front side ofthe mixer tools will be formed by an overall working face made up ofworking face sections of substantially flat form inclined at differentangles to the horizontal underface of the tools. Preferably, it will bearranged that the front side of the mixer tools presents, in the zoneadjacent the hub, an internal working face section which makes an anglewith the under face of the tools of to this angle being open towards thedirection of rotation. Furthermore, the front side of the mixer toolscan be provided in the neighbourhood of the external end with anexternal working face section which makes an angle with the underface ofthe tools, of about 45, this angle being open towards the direction ofrotation. In accordance with yet another embodiment, the mixer tool isfurther provided with an intermediate working face forming a transitionfrom the internal to the external workingface sections, saidintermediate face having an inclination corresponding substantially tothat of the external working face section. This kind of design of themixer tools gives them a substantially better displacement effectcoupled with reduced flow resistance, so that the material being mixed,while developing less frictional heat, is given a flow velocity which isrelatively high compared with the peripheral speed of the mixer tools.Coupled with reduced drive power for the operation of the mixer tools,the high flow velocity of the material being mixed gives rise toimproved heat transfer between it and the internal wall of the mixervessel, so that overall the mixer achieves a cooling performance whichis considerably superior to that of previously known mixers.

An embodiment of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a view in a partially cut-away overall side elevation of amixer-cooler in accordance with the invention;

FIG. 2 is a partially cut-away plan view of the mixercooler of FIG. 1;

FIG. 3 is an overall end elevation of the mixer-cooler shown in FIGS. 1and 2;

FIG. 4 is a view partially in side elevation and partially in verticalsection of the mixing vessel of the mixer cooler shown in FIGS. 1 to 3;

FIG. 5 is a schematic view in horizontal section of the flow duct in thecavity of the double wall of the mixer vessel, in the neighbourhood ofthe base thereof;

FIG. 6 is a view in plan of a mixer tool set;

FIGS. 7 and 8 are side elevations relating to FIG. 6; and

FIGS. 9 to 11 are simplified sectional illustrations through a mixertool, the sections being taken respectively on the lines IX, X and XI ofFIG. 7.

The mixer illustrated in the drawing consists of a mixing vessel 1 inwhich a set 2 of mixer tools is designed to rotate. The mixing vessel 1,set up upon a machine bed 3, has a substantially circular cylindricalform. The vessel axis 4 corresponds with the axis of rotation of the setof mixer tools. The top of the mixer vessel 1 is closed off by a coveror the like. To fill the internal cavity 5 of the mixer vessel 1, aclosable hatchway 6 is provided. The removal of the cooled mixedmaterial from the interior 5 is carried out through a lateral dischargeorifice which can be closed and opened by means of a device 7. Adelivery connection 8 of the cooling air blower 9 by means of whichcooling air or some other suitable cooling gas is blown into theinterior 5 of the vessel 1 during operation opens tangentially into theinterior 5 of the mixer vessel 1. On the cover 10, which contains an airvent orifice 11, there is a filter dome 12 where any particles of thematerial being mixed, which have been entrained by the exiting air, arefiltered out and retained. To drive the mixer tool set 2, an electricmotor 13 housed in the machine bed 3, is provided, this motor, through atransmission 14 rotating a vertical drive shaft 15 whose top end extendsthrough the vessel base. On the top end of the drive shaft a hub 16 issecured and above this a guide cone or spinner 17. This guide cone 17serves to prevent deposits of material collecting at the centre of themixing vessel 1, and rapidly returns material entering the central zoneof the vessel, to the field of operation of the mixer tool set 2.

The mixer vessel 1 has a double wall, the outer wall being designated 18and the inner wall 19. The double wall includes the sidewall and baseportions of the mixer vessel 1. The cavity 20 between the walls conductsa coolant, e.g. water, which is injected near to the centre of the basezone and is discharged from the upper cavity level with the rim of thevessel. The coolant discharge is indicated at 21 and comprises adischarge orifice 22 formed in the external wall 18. The coolant inlet(FIG. 4) consists of an inlet connection 23, in the external base wall18 of the vessel 1. It goes without saying that this coolant inletconnection 23 is connected in an appropriate manner to the coolantsource, the injection of the coolant being carried out and controlled bymeans of a pump, for example, which has not been shown.

The design of the mixing vessel 1 can be seen particularly clearly inFIGS. 4 and 5. In the base of the mixer vessel 1, there is providedfirst of all a central opening 24 to pass the drive shaft 15. Thisopening 24 is surrounded by an annular element 25 which closes off thecavity 20 between the base-side internal wall 19 and external wall 18'from the opening 24. The base side internal wall 19' of the mixingvessel 1 seats against a ring-mounting 26 arranged concentrically withthe ring element 25, which mounting extends through the base cavity 20'and projects downwards beyond the baseside external wall 18'. By meansof this ring mounting 26, the mixer vessel 1 as a whole is supportedupon the machine bed 3. In order to connect that zone 20' of the cavity,which is located concentrically inside the ring mounting 26, with thatzone 20" of the cavity located concentrically outside, a radicaltransfer orifice 26' is provided in the ring mounting 26. The cavity 20between the ring elements 25 and 26 is subdivided by a guide baffle 27disposed concentrically vis-a-vis the vessel axis 4. This guide baffle27, which, like the other guide baffles to be described later, mayconsist of bar material of round or rectangular (e.g. square)crosssection, and is in sealing contact both with the internal wall andwith the external wall of the double-walled vessel, defines a flow ductsection 28 in relation to the inner ring element 25, and in relation tothe ring mounting 26 defines a flow duct section 29. The two ductsections 28, 29 communicate with one another through a transfer orifice30. This transfer orifice 30 is disposed diametrally opposite the inletconnection 23 (considering the vessel axis 4.) That zone 20" of thecavity between internal and external walls of vessel 1, which extendsconcentrically outside the ring mounting 26 in the base zone of thevessel, is in turn subdivided by guide baffles 31,32,33 into ductsections 34, 35, 36 and 37. In accordance with a concentric arrangementof the guide baffles 31,32 and 33, the duct sections 34,'

35, 36 and 37 are also disposed concentrically vis-a-vis the vessel axis4. As FIG. 5 shows more clearly, a racial baffle 38 is provided in thecavity 20" baffle 38 extending from the ring mounting 26 and isconnected to one end 31', 32', 33' of each of the guide baffles 31, 32,33. The other ends 31", 32" and 33" of the concentric guide baffles 31,32 and 33 respectively, in each case terminate at an interval before theradial guide baffle 38, thus forming radial transfer orifices 39, 40 and41 respectively. This kind of design means that each duct section 34,35, 36 and 37 extends through a centrally subtended angle of around 360,each duct section communicating with its particular adjacent, radiallyouter section, through the associated transfer orifice. The ductsections 34, 35, 36 and 37 are connected in series with one another fromthe flow point of view so that the coolant, as it passes through thecoolant jacket, is forced to follow a labyrinthine path. This flow pathof the coolant entering through the transfer orifice 26 in the ringmounting 26, into the duct section 34, has been indicated in FIG. 5, towhich express reference is now made, by the curved arrows in suchfigure. These show how the flow direction is reversed from one ductsection to the next, in each case after the coolant has traversed asubtended angle of around 360. The flow is somewhat different in thecase of the specially designed vessel base in the zone concentricallywithin the ring mounting 26, as the arrows in FIG. 5 indicate. In thiscentral, inner zone 20' of the cavity, there is a change in flowdirection after a centrally subtended angle of only 180.

After flowing through the duct sections 28, 29 in the zone 20 of thecavity and then flowing through the duct sections 34, 35, 36 and 37 inthe zone 20" in the cavity, the coolant enters the side wall Zone 20 ofthe cavity 20 between the external 18 and internal l9 walls of the mixervessel 1. This cavity 20" is in turn subdivided by horizontal guidebaffles arranged coaxially at intervals, one above the other, thebaffles being designated 42, 43, 44, 45, 46 and 47 in FIG. 4. Theseguide baffles define between them duct sections 48, 49, 50, 51, 52 and53 of which the duct section 53 is closed off at the top by an element54 which forms the top closure of the cavity The duct sections 48 to 53are in turn connected in series with one another so that coolant leavingthe duct section 33 and entering the duct section 48 through a transferorifice (not shown) near the radial guide baffle 38, first of all flowsthrough the duct section 48, then the duct section 49, and so on, untilit flows out from duct section 53, at 22. In order also to create a flowpattern in the sidewall of the vessel which fundamentally coincides withthat in the external base zone of the vessel, the guide baffles 42 to 47are likewise in each case connected at one end to an axial guide baffle55 whilst their particular other ends termi nate at a certain distanceshort of this. For example, the guide baffle 44 is attached by its end44, the guide baffle 45 by its end 45', and the guide baffle 46 by itsend 46, to the axial guide baffle 55 in the cavity 20". Furthermore, forexample the ends 44", 45" and 46" of the guide baffles 44, 45, 46respectively, are arranged at an interval from the radial guide baffle55 in order to define in relation thereto transfer orifices 56, 57 and58, respectively. The flow pattern is once again indicated by arrows inthe cut-away part of FIG. 4.

Through the described design, an overall flow passage constituted by theduct sections 28, 29 34, 35, 36, 37. 48, 49, 50, SI, 52 and 53 isformed; such flow passage extends from the coolant inlet at 23 to thecoolant discharge at 22 and causes the coolant to follow a clearlydefined, labyrinthine path which uniformly covers the entire vesselwall. The guide baffles can have a mutual disposition such as to ensurethat the flow duct between the inlet at 23 and the discharge at 22 hasthe same flow cross-sectional area throughout, so that the flow velocityof the coolant remains the same throughout the entire coolant passage.Instead of this, however, it is possible, by suitable choice of the flowcrosssectional area in individual duct sections, locally temporarily toincrease or comparatively reduce the coolant flow velocities. This isuseful at points in the vessel where, because of the incidence ofmaterial being mixed, there is particularly intense heat development,since it permits the more effective dissipation of the heat transferredto the internal wall 19 of the vessel. Overall, the flow cross-sectionalarea of the flow duct is relatively small so that even at relatively lowcoolant flow rates, relatively high flow velocities can be achievedthere. By controlling the coolant pump performance, it is possiblefurthermore to increase or reduce the heat exchange between the internalwall 19 of the vessel and the coolant.

The set 2 ofmixer tools 59, 60, 61, illustrated in more detail in FIGS.6 to 11, cooperates in a special way to assist the mixer-cooler toachieve a particularly high cooling performance. The mixer tools 59, 60,61 initially have their front side (considered in the direction ofrotation) running tangentially into the circumferential surface of thehub 16. Through this disposition on the front side of the mixer tools,the material being mixed is imparted a direct radial component ofmovement, corresponding with the direction of the centrifugal force, sothat in contrast to the case which would obtain with radial alignment ofthe mixer tools, where the material would simply be given acircumferential component of movement, the radial motion being dueentirely to centrifugal force in that case, the material acquires aparticularly strong outwardly accelerated flow.

The front side of the mixer tools, furthermore, present a total workingface which is made up of individual working face sections. These workingface sections have differing inclinations vis-a-vis the horizontalunderface of the tools. In the example illustrated, the overall workingface at the front side of the tool. is made up of three sections (62, 63and 64). The steep face 62 includes the zone adjoining the hub 16 andmakes an angle of between and with the base 65, which angle is opentowards the direction of rotation. By means of the thus relativelysteeply angled, substantially flat, internal working face 62, thematerial being mixed is given a circumferential and outward flow, theupward component being so slight that it is sufficient merely to preventclogging at the base but does not prevent an effective upward motion onthe part of the material.

The face section 64 includes the external end zone of the front side ofthe mixer tools, and makes an angle of about 45 with the base of thetools, which angle is open in the direction of rotation. This externalface sec tion is enlarged scoop fashion and has a dimension in thedirection of the axis 4 which is 2 to 3 times the correspondingdimension of the face section 62. The relatively small angle ofinclination means that the material being mixed is given a particularlystrong upward acceleration. The face section 63 is an intermediate faceconstituting a transition from the inner face section 62 to the outerface section 64. It is substantially triangular in shape with the apexlocated at the front edge of the mixer tool at the level of the base andat the transition between internal and external face sections. Itsinclination corresponds substantially to that of the external facesection 64. As far as the disposition of the lines of intersection orjunction edges, between the face section 62 and the face section 64, onthe one hand, and the intermediate face 63, on the other, is concerned,reference should be made to FIGS. 6 to 8. I

The face section 64 tapers upwards in width and in that of its zonesdisposed towards the internal wall of the mixer vessel 1, terminates ina rounded outer edge. Considered in relation to the front side of themixer tools, the inner working face section extends over about 75percent of the total tool length, measured at the front edge at thelevel of the base of the tool. The working face section 63 commences atabout the midlength of the mixer tool and, measured at its top terminaledge, extends over about 25 to 30 percent of the total tool length.Measured at the front edge ofthe tool, approximately at the level of thebase, the face section 64 occupies about 25 percent of the total lengthof the tool and reduces, up to its upper terminal edge, to about 20percent of the total tool length. The radius of the hub 16 has a ratioof about 1:3 to the total length of the tool measured at the lattersfront side. At the base, the cone or spinner 17 has a diametercorresponding to the diameter of the hub 16. The height of the cone isabout twice the axial height of the face section 64.

A particularly cheap embodiment of the mixer tools is one in which theseare formed as a hollow welded fabrication, embodying a tube section 66as the main mounting. This tube section 66, with that of its areaswhich, in the direction of rotation, is located in the lee,

forms the rearward trailing edge of the mixer tool. The top and basesides of the tool as well as the working face sections, are formed bypieces of plate which are welded both to the tube section 66 and to oneanother.

It goes without saying that the external surfaces of the tool aremachined to form a smooth surface, the weld seams being polished inparticular.

I claim:

1. A mixer-cooler, comprising a mixing vessel into which a substance tobe mixed may be introduced, a mixing tool arranged therein and designedto rotate about a vertical axis, the mixing vessel having internal andexternal walls defining between them a cavity through which a coolantflows, the mixing tool being configured to propel the substance radiallyoutwardly along the base of the vessel and thereafter up the internalwall of the vessel, a coolant inlet in the base of the vessel and acoolant discharge near the upper rim of the vessel, guide elements insaid space to guide the coolant on its passage between the coolant inletand the coolant discharge in a direction generally corresponding to thedirection of propulsion of the substance, the guide elements subdividingthe cavity of the double wall into a coolant duct extending inlabyrinthine fashion from the coolant inlet to the coolant discharge inat least the radially outer zone of the base-side cavity of the doublewall, the flow duct consisting of concentric duct sections formed byguide baffles extending substantially concentrically to the vessel axiseach of which duct sections extends over a centrally subtended angle ofaround 360 and communicates through a radial transfer orifice with theparticular adjacent, radially outer duct section in a seriesdisposition, the flow duct in the side wall zone ofthe double wall beingmade up of duct sections defined by guide baffles arranged coaxially, atintervals, parallel one above the other in the cavity, each of said ductsections extending over a centrally subtended angle of 360 andcommunicating through an axial transfer orifice with the particularadjacent, axially upper duct section in a series disposition.

2. A mixer-cooler as claimed in claim 1 in which the flow duct presentsa substantially unchanged flow cross-sectional area in the zone betweenthe coolant inlet and the coolant discharge.

3. A mixer-cooler as claimed in claim 1, in which the flowcross-sectional area of the flow duct has locally different dimensionsbetween the coolant inlet and the coolant discharges.

4. A mixer-cooler as claimed in claim 3, in which the guide baffles aremade of bar material.

5. A mixer-cooler as claimed in claim 4, comprising a central driveshaft having a hub, and a multiple set of mixer tools identical to oneanother and attached to the central hub, the front side, considered inthe direction of rotation, of the mixer tools having a tangentialalignment to the circumference of the hub.

6. A mixer-cooler as claimed in claim 5, in which the front side of themixer tool has an overall working face made up of working face sectionswhich are substantially flat and inclined at different angles to thehorizontal base of the tool; said front side of the mixer tools in thezone adjacent the hub presents an inner face section makingan angle ofbetween and with the base of the tools, which angle is open towards thedirection of rotation; and in which the front side of the mixer tools,in the neighborhood of the external end, presents an external facesection making an angle of about 45 with the tool base which angle isopen towards the direction of rotation.

7. A mixer-cooler as claimed in claim 6 including an intermediate facesection forming a transition between the inner and external facesections, which intermediate section has an inclination substantiallycorresponding tothat of the outer face section.

8. A mixer-cooler as claimed in claim 7, in which the intermediate facesection has substantially the shape of a triangle whose apex is locatedat the front edge of the mixer tool at the level of the base and at thetransition between inner and external face sections.

9. A mixer-cooler as claimed in claim 8, in which the external facesection has a dimension which substantially exceeds the dimension of theinner face section in the direction of the vessel axis. 4

10. A mixer-cooler as claimed in claim 8, characterised in that themixer tools take the form of a welded hollow fabrication made up of atube section as the main mounting, and pieces of plate.

1. A mixer-cooler, comprising a mixing vessel into which a substance tobe mixed may be introduced, a mixing tool arranged therein and designedto rotate about a vertical axis, the mixing vessel having internal andexternal walls defining between them a cavity through which a coolantflows, the mixing tool being configured to propel the substance radiallyoutwardly along the base of the vessel and thereafter up the internalwall of the vessel, a coolant inlet in the base of the vessel and acoolant discharge near the upper rim of the vessel, guide elements insaid space to guide the coolant on its passage between the coolant inletand the coolant discharge in a direction generally corresponding to thedirection of propulsion of the substance, the guide elements subdividingthe cavity of the double wall into a coolant duct extending inlabyrinthine fashion from the coolant inlet to the coolant discharge inat least the radially Outer zone of the base-side cavity of the doublewall, the flow duct consisting of concentric duct sections formed byguide baffles extending substantially concentrically to the vessel axiseach of which duct sections extends over a centrally subtended angle ofaround 360* and communicates through a radial transfer orifice with theparticular adjacent, radially outer duct section in a seriesdisposition, the flow duct in the side wall zone of the double wallbeing made up of duct sections defined by guide baffles arrangedcoaxially, at intervals, parallel one above the other in the cavity,each of said duct sections extending over a centrally subtended angle of360* and communicating through an axial transfer orifice with theparticular adjacent, axially upper duct section in a series disposition.2. A mixer-cooler as claimed in claim 1 in which the flow duct presentsa substantially unchanged flow cross-sectional area in the zone betweenthe coolant inlet and the coolant discharge.
 3. A mixer-cooler asclaimed in claim 1, in which the flow cross-sectional area of the flowduct has locally different dimensions between the coolant inlet and thecoolant discharges.
 4. A mixer-cooler as claimed in claim 3, in whichthe guide baffles are made of bar material.
 5. A mixer-cooler as claimedin claim 4, comprising a central drive shaft having a hub, and amultiple set of mixer tools identical to one another and attached to thecentral hub, the front side, considered in the direction of rotation, ofthe mixer tools having a tangential alignment to the circumference ofthe hub.
 6. A mixer-cooler as claimed in claim 5, in which the frontside of the mixer tool has an overall working face made up of workingface sections which are substantially flat and inclined at differentangles to the horizontal base of the tool; said front side of the mixertools in the zone adjacent the hub presents an inner face section makingan angle of between 75* and 85* with the base of the tools, which angleis open towards the direction of rotation; and in which the front sideof the mixer tools, in the neighborhood of the external end, presents anexternal face section making an angle of about 45* with the tool basewhich angle is open towards the direction of rotation.
 7. A mixer-cooleras claimed in claim 6 including an intermediate face section forming atransition between the inner and external face sections, whichintermediate section has an inclination substantially corresponding tothat of the outer face section.
 8. A mixer-cooler as claimed in claim 7,in which the intermediate face section has substantially the shape of atriangle whose apex is located at the front edge of the mixer tool atthe level of the base and at the transition between inner and externalface sections.
 9. A mixer-cooler as claimed in claim 8, in which theexternal face section has a dimension which substantially exceeds thedimension of the inner face section in the direction of the vessel axis.10. A mixer-cooler as claimed in claim 8, characterised in that themixer tools take the form of a welded hollow fabrication made up of atube section as the main mounting, and pieces of plate.